The NF-kappa B cascade is important in Bcl-xL expression and for the anti-apoptotic effects of the CD28 receptor in primary human CD4+ lymphocytes

4-1BB promotes the survival of CD8+ T lymphocytes by increasing expression of Bcl-xL and Bfl-1

4-1BB, a T cell costimulatory receptor, prolongs CD8(+) T cell survival. In these studies, 4-1BB stimulation was shown to increase expression of the antiapoptotic genes bcl-x(L) and bfl-1 via 4-1BB-mediated NF-kappaB activation. This signaling pathway was specifically inhibited by PDTC and was different from the pathways that enhanced CD8(+) T cell proliferation. The results suggest a role for the antiapoptotic activities of Bcl-x(L) and Bfl-1 proteins in 4-1BB-mediated CD8(+) T cell survival in vivo.

Overexpression of FKBP51 in idiopathic myelofibrosis regulates the growth factor independence of megakaryocyte progenitors

Idiopathic myelofibrosis (IMF) is a chronic myeloproliferative disorder characterized by megakaryocyte hyperplasia and bone marrow fibrosis. Biologically, an autonomous megakaryocyte growth and differentiation is noticed, which contributes to the megakaryocyte accumulation. To better understand the molecular mechanisms involved in this spontaneous growth, we searched for genes differentially expressed between normal megakaryocytes requiring cytokines to grow and IMF spontaneously proliferating megakaryocytes. Using a differential display technique, we found that the immunophilin FKBP51 was 2 to 8 times overexpressed in megakaryocytes derived from patients' CD34(+) cells in comparison to normal megakaryocytes. Overexpression was moderate and confirmed in 8 of 10 patients, both at the mRNA and protein levels. Overexpression of FKBP51 in a UT-7/Mpl cell line and in normal CD34(+) cells induced a resistance to apoptosis mediated by cytokine deprivation with no effect on proliferation. FKBP51 interacts with both calcineurin and heat shock protein (HSP)70/HSP90. However, a mutant FKBP51 deleted in the HSP70/HSP90 binding site kept the antiapoptotic effect, suggesting that the calcineurin pathway was responsible for the FKBP51 effect. Overexpression of FKBP51 in UT-7/Mpl cells induced a marked inhibition of calcineurin activity. Pharmacologic inhibition of calcineurin by cyclosporin A mimicked the effect of FKBP51. The data support the conclusion that FKBP51 inhibits apoptosis through a calcineurin-dependent pathway. In conclusion, FKBP51 is overexpressed in IMF megakaryocytes and this overexpression could be, in part, responsible for the megakaryocytic accumulation observed in this disorder by regulating their apoptotic program.

Anti-inflammatory lipid mediators and insights into the resolution of inflammation

The pro-inflammatory signalling pathways and cellular mechanisms that initiate the inflammatory response have become increasingly well characterized. However, little is known about the mediators and mechanisms that switch off inflammation. Recent data indicate that the resolution of inflammation is an active process controlled by endogenous mediators that suppress pro-inflammatory gene expression and cell trafficking, as well as induce inflammatory-cell apoptosis and phagocytosis, which are crucial determinants of successful resolution. This review focuses on this emerging area of inflammation research and describes the mediators and mechanisms that are currently stealing the headlines.

OX40 signaling renders adult T-cell leukemia cells resistant to Fas-induced apoptosis

We reported previously that OX40, a member of the tumor necrosis factor receptor family, is expressed constitutively on fresh leukemia/lymphoma cells isolated from patients with adult T-cell leukemia (ATL). In this study, we tested whether OX40 signaling affects the Fas-mediated apoptosis of fresh ATL cells isolated from 7 patients (3 acute type, 3 chronic type, and 1 smoldering type). In all these patients, the coculture of ATL cells with MMCE/OX40 ligand gp34, a stable human gp34 transfectant of a mouse epithelial cell line, resulted in a decrease in the percentage of apoptotic cells after treatment with anti-Fas monoclonal antibody, compared to coculture with MMCE/mock controls. Similar findings were obtained in OX40(+)- human T-cell leukemia virus type I-transformed T-cell lines. To elucidate the molecular mechanism of this phenomenon, we used Kit225/OX40, a stable OX40 transfectant of an IL-2-dependent T-cell line, and its deletion mutant, Kit225/del-OX40, in which the intracytoplasmic domain of OX40 had been deleted. Coculture with MMCE/gp34 inhibited the apoptosis of Kit225/OX40, but Kit225/del-OX40 apoptosis was hardly affected. These results suggest that ATL cells may escape Fas-mediated destruction of the immune system through OX40 signaling.

Potential methods to circumvent blocks in apoptosis in lymphomas

We review our current understanding of the molecular determinants and mechanisms of lymphocyte apoptosis and identify the key regulators of these death-signaling pathways. In addition, we describe the key molecular aberrations that underlie the resistance of lymphomas to conventional therapy, and highlight the enormous promise of potential therapeutic strategies that could circumvent or overcome these genetic impediments to apoptosis.

It's all Rel-ative: NF-kappaB and CD28 costimulation of T-cell activation

Costimulatory signals complement or modify the signals provided to a lymphocyte through antigen receptors. For productive T-cell activation, the CD28 molecule is apparently the most important, although not the only, costimulatory receptor. CD28 can provide a signal that is at least partially distinct from that delivered by the T cell receptor (TCR)-CD3 complex. Several lines of evidence indicate that the nuclear factor (NF)-kappaB pathway is perhaps the most relevant biochemical or transcriptional target for the costimulatory activity of CD28. Although many questions remain, recent years have witnessed significant progress in understanding the signal transduction pathways leading from the TCR and CD28 to Rel/NF-kappaB-dependent transcription.

Recruitment of phosphatidylinositol 3-kinase to CD28 inhibits HIV transcription by a Tat-dependent mechanism

Activation through the TCR and the costimulatory molecule CD28 influences the susceptibility of T cells to HIV-1 infection and regulates proviral gene expression. Signaling events initiated by CD28 that directly impact HIV-1 transcription have not been fully characterized. T cell lines expressing CD8alpha/28 chimeric receptors containing a mutation in tyrosine 173 to phenylalanine, which inhibits the recruitment of phosphatidylinositol 3-kinase (PI3K) to CD28, expressed higher levels of HIV-1 following T cell activation. Whereas constitutively active PI3K decreased provirus transcription, inhibiting endogenous PI3K with specific inhibitors or by overexpressing PTEN phosphatase enhanced HIV-1 expression. PI3K-dependent inhibition required the viral Tat protein and a trans activation response region element. Tat pull-down and coimmunoprecipitation experiments indicate that PI3K affects the formation of the Tat-associated kinase trans-activating complex. These studies demonstrate that PI3K negatively impacts HIV-1 transcription and that Tat activity is sensitive to T cell signaling events.

NF-kappaB family of transcription factors: central regulators of innate and adaptive immune functions

Transcription factors of the Rel/NF-kappaB family are activated in response to signals that lead to cell growth, differentiation, and apoptosis, and these proteins are critical elements involved in the regulation of immune responses. The conservation of this family of transcription factors in many phyla and their association with antimicrobial responses indicate their central role in the regulation of innate immunity. This is illustrated by the association of homologues of NF-kappaB, and their regulatory proteins, with resistance to infection in insects and plants (M. S. Dushay, B. Asling, and D. Hultmark, Proc. Natl. Acad. Sci. USA 93:10343-10347, 1996; D. Hultmark, Trends Genet. 9:178-183, 1993; J. Ryals et al., Plant Cell 9:425-439, 1997). The aim of this review is to provide a background on the biology of NF-kappaB and to highlight areas of the innate and adaptive immune response in which these transcription factors have a key regulatory function and to review what is currently known about their roles in resistance to infection, the host-pathogen interaction, and development of human disease.

Regulation of the gadd45beta promoter by NF-kappaB

In addition to coordinating immune and inflammatory responses, NF-kappaB/Rel transcription factors control cell survival. The NF-kappaB antiapoptotic function is crucial to oncogenesis, cancer chemoresistance, and to antagonize tumor necrosis factor (TNF) receptor-induced killing. Recently, we have shown that the suppression of the c-Jun-N-terminal kinase (JNK) cascade is a pivotal protective mechanism by NF-kappaB, and that this suppression involves the upregulation of gadd45beta/myd118. Induction of gadd45beta by stress and cytokines requires NF-kappaB; however, the regulatory mechanisms underlying this induction are not known. Here, we report that, in HeLa cells, the NF-kappaB subunit RelA is sufficient to activate gadd45beta expression, whereas Rel and p50 are not. Activation of gadd45beta by RelA depends on three kappaB elements at positions -447/-438 (kappaB-1), -426/-417 (kappaB-2), and -377/-368 (kappaB-3) of the gadd45beta promoter. Each of these sites binds to NF-kappaB complexes in vitro, and is required for optimal promoter transactivation. The data establish the direct participation of NF-kappaB in the regulation of Gadd45beta, thereby providing important mechanistic insights into the control of apoptosis by the transcription factor.

T-cell activation through the antigen receptor. Part 2: role of signaling cascades in T-cell differentiation, anergy, immune senescence, and development of immunotherapy

Part 2 of this review on cellular activation by the T-cell antigen receptor (TCR) will highlight how TCR signaling pathways are adapted to achieve specific biologic outcomes, including different states of T-cell differentiation and the induction of T-cell tolerance. We will also explore how treatment with altered peptide ligands affects TCR signaling to change T-cell differentiation or to induce an anergy state. These changes are accomplished through alteration of protein tyrosine kinase activity, the stoichiometry of phosphorylation of immunoreceptor tyrosine-based activation motifs, intracellular free ionized calcium flux, mitogen-activated protein kinase activity, and transcriptional activation of key cytokine promoters. The CTLA-4 plays an important role in the induction and maintenance of anergy. The second theme will highlight how altered TCR signal transduction, including changes in the compartmentalization of signaling components at the TCR synapse, contributes to decreased T-cell activation during immune senescence. Finally, we will illustrate how the molecular details of TCR activation can be used to modify the function of the immune system. This includes a description of the mechanism of action of altered peptide ligands, CTLA-4Ig, and pharmacologic inhibitors of mitogen-activated protein kinases, nuclear factor kappaB, and protein kinase C cascades.

Protein kinase C and AKT/protein kinase B in CD4+ T-lymphocytes: new partners in TCR/CD28 signal integration

T-cell biological responses appear to involve the complex interaction of T-cell surface receptors, intracellular signaling molecules and the cytoskeleton. Both the serine/threonine protein kinase families protein kinase C (PKC) and protein kinase B or RAC-PK (AKT/PKB) have been implicated in signal transmission leading to activation, differentiation as well as cellular survival of T-lymphocytes. The PKC gene family consists of nine diverse isotypes (PKC alpha, beta, gamma, delta, epsilon, xi, eta, theta; and iota), the AKT/PKB gene family includes three kinases (AKT1/PKB alpha, AKT2/PKB beta, AKT3/PKB gamma). Here, we attempt to summarize the regulation as well as downstream signaling pathways of PKC and AKT/PKB isotypes, that may act additive in TCR/CD28 induced proliferation and survival of peripheral CD4+ T-lymphocytes.

Constitutive nuclear factor-kappaB activity preserves homeostasis of quiescent mature lymphocytes and granulocytes by controlling the expression of distinct Bcl-2 family proteins

Constitutive nuclear factor kappaB (NF-kappaB) activity protects quiescent mature immune cells from spontaneous apoptosis. Here, we examined whether NF-kappaB exerts its antiapoptotic function in these cells through the control of Bcl-2 family proteins. Specific pharmacologic inhibitors of NF-kappaB were used to achieve total NF-kappaB inactivation in quiescent human blood lymphocytes, granulocytes, and monocytes. NF-kappaB inhibition induced drastic lymphocyte and granulocyte apoptosis, but only moderate monocyte apoptosis. T- and B-cell apoptosis was slow and associated with a gradual down-regulation of the prosurvival Bcl-2 family proteins Bcl-x(L) and Bcl-2, respectively. By contrast, granulocyte apoptosis was fast and accompanied by a rapid cellular accumulation of Bcl-x(S), the proapoptotic Bcl-x isoform that is generated from alternative splicing of the bcl-x pre-mRNA. Finally, antisense bcl-x(L) and bcl-2 knockdown in T and B cells, respectively, and induction of Bcl-x(S) expression in granulocytes through antisense oligonucleotide-mediated redirection of bcl-x pre-mRNA splicing were sufficient to induce significant apoptosis in these cells. Taken together, these results reveal that basal NF-kappaB activity preserves homeostasis of quiescent mature lymphocytes and granulocytes through regulation of distinct members of the Bcl-2 family. This study sheds light on the constitutive mechanisms by which NF-kappaB maintains defense integrity.

NF-kappa B as a therapeutic target in multiple myeloma

We have shown that thalidomide (Thal) and its immunomodulatory derivatives (IMiDs), proteasome inhibitor PS-341, and As(2)O(3) act directly on multiple myeloma (MM) cells and in the bone marrow (BM) milieu to overcome drug resistance. Although Thal/IMiDs, PS-341, and As(2)O(3) inhibit nuclear factor (NF)-kappaB activation, they also have multiple and varied other actions. In this study, we therefore specifically address the role of NF-kappaB blockade in mediating anti-MM activity. To characterize the effect of specific NF-kappaB blockade on MM cell growth and survival in vitro, we used an IkappaB kinase (IKK) inhibitor (PS-1145). Our studies demonstrate that PS-1145 and PS-341 block TNFalpha-induced NF-kappaB activation in a dose- and time-dependent fashion in MM cells through inhibition of IkappaBalpha phosphorylation and degradation of IkappaBalpha, respectively. Dexamethasone (Dex), which up-regulates IkappaBalpha protein, enhances blockade of NF-kappaB activation by PS-1145. Moreover, PS-1145 blocks the protective effect of IL-6 against Dex-induced apotosis. TNFalpha-induced intracellular adhesion molecule (ICAM)-1 expression on both RPMI8226 and MM.1S cells is also inhibited by PS-1145. Moreover, PS-1145 inhibits both IL-6 secretion from BMSCs triggered by MM cell adhesion and proliferation of MM cells adherent to BMSCs. However, in contrast to PS-341, PS-1145 only partially (20-50%) inhibits MM cell proliferation, suggesting that NF-kappaB blockade cannot account for all of the anti-MM activity of PS-341. Importantly, however, TNFalpha induces MM cell toxicity in the presence of PS-1145. These studies demonstrate that specific targeting of NF-kappaB can overcome the growth and survival advantage conferred both by tumor cell binding to BMSCs and cytokine secretion in the BM milieu. Furthermore, they provide the framework for clinical evaluation of novel MM therapies based upon targeting NF-kappaB.

Contrasting the roles of costimulation and the natural adjuvant lipopolysaccharide during the induction of T cell immunity

The requirements for circumventing tolerance induction in favor of memory T cell development are poorly understood. Although two signals (Ag and costimulation) are necessary to drive effective T cell clonal expansion, few memory T cells remain after the response wanes. The adjuvant LPS can increase numbers of long-lived Ag-specific T cells, but its mechanism of action is not understood. In this report, it is shown that LPS, when combined with two-signal stimulation, profoundly enhances T cell survival in vivo. This survival does not appear to be dependent on the cytokines TNF-alpha, IL-1 beta, IL-6, and IFN-gamma, nor is it dependent on the transcription factor NF-kappa B. However, in vivo proliferation of NF-kappa B-deficient T cells was comparable to that of wild-type T cells, yet their early accumulation in the lymph nodes was severely reduced unless the mice were treated with LPS and an agonistic CD40 mAb. Most importantly, we found that activation of two different costimulatory signals, CD40 and OX40, could not substitute for LPS in rescuing T cells from peripheral deletion. Perhaps surprisingly, these data show that LPS delivers a qualitatively different signal than multiple costimulatory signals.

Aggregation of the high-affinity IgE receptor Fc(epsilon)RI on human monocytes and dendritic cells induces NF-kappaB activation

In contrast to mast cells and basophils, the high-affinity IgE receptor (Fc(epsilon)RI) on monocytes and dendritic cells (DC), including epidermal Langerhans cells, is not constitutively expressed and lacks the beta-chain. Fc(epsilon)RI is upregulated on Langerhans cells of atopic individuals, particularly in atopic dermatitis skin. Although Fc(epsilon)RI provides IgE-mediated antigen focusing on monocytes and DC/Langerhans cells, its relevance for cell activation remains elusive, and the transcription factors regulating Fc(epsilon)RI-induced genes are unknown. We show that NF-kappaB, known to regulate genes essential for inflammatory responses and DC differentiation and function, is activated upon Fc(epsilon)RI ligation in primary human monocytes and DC. In Langerhans cells isolated from epidermis, NF-kappaB activation is restricted to donors expressing high Fc(epsilon)RI amounts. Fc(epsilon)RI-induced NF-kappaB complexes in monocytes and DC contain p50 and p65, but no other NF-kappaB subunits despite increased RelB expression during differentiation. NF-kappaB activation is preceded by serine phosphorylation and degradation of its inhibitory protein IkappaB-alpha without involving other IkappaB proteins. Finally, we show that Fc(epsilon)RI ligation on monocytes and DC leads to synthesis and release of tumor necrosis factor-alpha and monocyte chemoattractant protein-1, which is decreased by two mechanistically distinct inhibitors of NF-kappaB activation. Thus NF-kappaB activation represents a novel mechanism by which Fc(epsilon)RI on monocytes and DC potentially controls inflammatory reactions.

T-cell activation through the antigen receptor. Part 1: signaling components, signaling pathways, and signal integration at the T-cell antigen receptor synapse

Part 1 of this review will highlight the basic components and signaling pathways by which the T-cell antigen receptor (TCR) activates mature extrathymic T cells. TCR signaling commences with an early wave of protein tyrosine kinase activation, which is mediated by the Src kinases Lck and Fyn, the 70-kd zeta-associated protein kinase, and members of the Tec kinase family. This early wave of protein tyrosine phosphorylation leads to the activation of downstream signaling pathways, including an increase in intracellular free calcium, protein kinase C, nuclear factor kappaB and Ras-mitogen-activated protein kinase activation. These pathways activate transcription factors, such as activator protein 1, nuclear factor of activated T cells, and Rel proteins, which ultimately lead to the expression of genes that control cellular proliferation, differentiation, anergy, or apoptosis. This review also describes how costimulatory receptors assist in signal transduction and assembly of macromolecular complexes at the TCR contact site with the antigen-presenting cell, also known as the immune synapse. These basic concepts of TCR signal transduction will be used in part 2 to explain how T-cell function can be altered by therapeutic targeting of TCR signaling components, as well as to explain modification of TCR signaling during T(H)1/T(H)2 differentiation, tolerance, and immune senescence.

Bcl-2, Bcl-xL and c-FLIP(L) potentially regulate the susceptibility of human peripheral blood monocyte-derived dendritic cells to cell death at different developmental stages

We examined the susceptibility of human monocyte-derived dendritic cells (DCs) to spontaneous and CD95-mediated cell death at different developmental stages. Time course experiments revealed that the susceptibility of mature dendritic cells (mDCs) to spontaneous cell death was significantly lower than that of immature dendritic cells (iDCs) in a long-term culture under cytokine-free conditions, and the treatment with GM-CSF rescued these cells from spontaneous cell death at the late culture period. iDCs and mDCs expressed similar levels of CD95 whereas both cell types were relatively resistant to CD95-mediated cell death. Antigen (Ag)-specific and nonspecific cognate interaction with T cells failed to cause cell death of iDCs and mDCs. iDCs constitutively expressed transcripts and intracellular products of Bcl-2 and Bcl-xL, but not cellular FLICE-inhibitory protein(long (c-FLIP(L)), while the increased expressions of Bcl-2, Bcl-xL and c-FLIP(L) were observed in mDCs. These results suggest that the selective expressions of Bcl-2, Bcl-xL and c-FLIP(L) may be involved in the difference in the susceptibility to cell death between iDCs and mDCs.

Molecular mechanisms underlying differential contribution of CD28 versus non-CD28 costimulatory molecules to IL-2 promoter activation

T cell costimulation via CD28 and other (non-CD28) costimulatory molecules induces comparable levels of [(3)H]TdR incorporation, but fundamentally differs in the contribution to IL-2 production. In this study, we investigated the molecular basis underlying the difference between CD28 and non-CD28 costimulation for IL-2 gene expression. Resting T cells from a mutant mouse strain generated by replacing the IL-2 gene with a cDNA encoding green fluorescent protein were stimulated with a low dose of anti-CD3 plus anti-CD28 or anti-non-CD28 (CD5 or CD9) mAbs. CD28 and non-CD28 costimulation capable of inducing potent [(3)H]TdR uptake resulted in high and marginal levels of green fluorescent protein expression, respectively, indicating their differential IL-2 promoter activation. CD28 costimulation exhibited a time-dependent increase in the binding of transcription factors to the NF-AT and NF-kappaB binding sites and the CD28-responsive element of the IL-2 promoter, whereas non-CD28 costimulation did not. Particularly, a striking difference was observed for the binding of NF-kappaB to CD28-responsive element and the NF-kappaB binding site. Decreased NF-kappaB activation in non-CD28 costimulation resulted from the failure to translocate a critical NF-kappaB member, c-Rel, to the nuclear compartment due to the lack of IkappaBbeta inactivation. These observations suggest that unlike CD28 costimulation, non-CD28 costimulation fails to sustain IL-2 promoter activation and that such a failure is ascribed largely to the defect in the activation of c-Rel/NF-kappaB.

Temporal segregation of 4-1BB versus CD28-mediated costimulation: 4-1BB ligand influences T cell numbers late in the primary response and regulates the size of the T cell memory response following influenza infection

In this report, we demonstrate that CD28(-/-) mice are severely impaired in the initial expansion of D(b)/NP366-374-specific CD8 T cells in response to influenza virus infection, whereas 4-1BB ligand (4-1BBL)(-/-) mice show no defect in primary T cell expansion to influenza virus. In contrast, 4-1BBL(-/-) mice show a decrease in D(b)/NP366-374-specific T cells late in the primary response. Upon secondary challenge with influenza virus, 4-1BBL(-/-) mice show a decrease in the number of D(b)/NP366-374-specific T cells compared to wild-type mice such that the level of the CD8 T cell expansion during the in vivo secondary response is reduced to the level of a primary response, with concomitant reduction of CTL effector function. In contrast, Ab responses, as well as secondary CD4 T cell responses, to influenza are unaffected by 4-1BBL deficiency. Thus, CD28 is critical for initial T cell expansion, whereas 4-1BB/4-1BBL signaling affects T cell numbers much later in the response and is essential for the survival and/or responsiveness of the memory CD8 T cell pool.

NF-kappaB at the crossroads of life and death

The choice between life and death is one of the major events in regulation of the immune system. T cells that specifically recognize viral or bacterial antigens are selected to survive and proliferate in response to infection, whereas those that are self-reactive are eliminated via apoptosis. Even the survival of alloreactive T cells requires their proper costimulation and, when infection subsides, the activated T cells are eliminated. A major regulator of such life or death decisions is the transcription factor NF-kappaB. However, NF-kappaB cannot function alone. A variety of mechanisms exist to modulate its activity and thereby affect the ultimate outcome of a cell's fate.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

Regulation of constitutive STAT5 phosphorylation in acute myeloid leukemia blasts

In the present study, we examined the underlying mechanism, which causes the constitutive tyrosine phosphorylation of signal transducer and activator of transcription 5 (STAT5) in acute myeloid leukemia (AML) blasts. Constitutive STAT5 phosphorylation was observed in 18 of 26 (69%) patients with AML. The constitutive STAT5 phosphorylation was caused by different mechanisms. In the majority of the investigated cases (71% (12 of 17)) constitutive STAT5 phosphorylation was associated with autophosphorylation of the type III receptor tyrosine kinase Flt3. In 47% (eight of 17) of these cases autophosphorylation of Flt3 coincided with tandem duplications of the Flt3 gene, resulting in constitutive phosphorylation of the receptor, while 24% (four of 17) of the cases demonstrated STAT5 phosphorylation and Flt3 autophosphorylation without mutations. In addition, a subset of AML cases (29% (five of 17)) had no autophosphorylation of the Flt3 receptor, but demonstrated constitutive STAT5 phosphorylation, which was partly due to autocrine growth factor production. All AML cases with high STAT5 and Flt3 phosphorylation demonstrated, in general, a lower percentage of spontaneous apoptosis, compared to AML blasts with no spontaneous STAT5 phosphorylation. Addition of the receptor tyrosine III kinase inhibitor AG1296 strongly inhibited STAT5 phosphorylation and enhanced the percentage of apoptotic cells without modulating the Bcl-xl protein levels. These data indicate that in the majority of AML cases the constitutive STAT5 phosphorylation is caused by Flt3 phosphorylation mostly due to mutations in the receptors and associated with a low degree of spontaneous apoptosis.

How melanoma cells evade trail-induced apoptosis

At the doses used clinically, chemotherapy is believed to kill melanoma by a final common 'mitochondrial' pathway that leads to apoptosis. Similarly, several natural defence mechanisms kill melanoma by the same pathways. A corollary to the latter is that survival of melanoma in the host is due to the development of anti-apoptotic mechanisms in melanoma cells. What are these mechanisms? And how might we bypass them to improve the treatment of melanoma?

The contribution of NF-kappa B activity to spontaneous proliferation and resistance to apoptosis in human T-cell leukemia virus type 1 Tax-induced tumors

Human T-cell leukemia virus type I is the etiologic agent of adult T-cell leukemia/lymphoma. The Tax protein of this virus is thought to contribute to cellular transformation and tumor development. In this report, we have used a Tax transgenic mouse model of tumorigenesis to study the contribution of nuclear factor (NF)-kappa B activity to spontaneous tumor cell proliferation and resistance to apoptosis. We have demonstrated elevated expression levels of NF-kappa B--inducible cytokines, including interleukin (IL)-6, IL-10, IL-15, and interferon (IFN)-gamma, in freshly isolated primary tumors from Tax transgenic mice. Inhibitors of NF-kappa B activity, sodium salicylate and cyclopentenone prostaglandins (prostaglandin A(1) and 15-deoxy-Delta(12,14)-prostaglandin J(2)), blocked spontaneous proliferation of Tax transgenic mouse spleen cells. In addition, Tax-induced tumor cells, which are resistant to irradiation-induced apoptosis, became sensitive to apoptosis in the presence of sodium salicylate and prostaglandins. These results strongly suggest that Tax-mediated induction of NF-kappa B activity contributes to tumorigenesis in vivo. (Blood. 2001;98:1200-1208)

Neurotrophins prevent HIV Tat-induced neuronal apoptosis via a nuclear factor-kappaB (NF-kappaB)-dependent mechanism

HIV-1 associated dementia is thought to be caused by neuronal damage and death in response to the production of soluble neurotoxic factors by virally infected mononuclear phagocytes. These neurotoxins include HIV-1 Tat. The ability of neurotrophins to promote cell survival prompted us to examine whether neurotrophins might also be capable of opposing the pro-apoptotic effects of Tat. Here, we show that Tat-induced neuronal apoptosis in primary cultures of rat cerebellar granule cells and in neuronally differentiated human SK-N-MC cells is profoundly inhibited by brain-derived neurotrophic factor, nerve growth factor and activity-dependent neurotrophic factor nonamer peptide. These neurotrophins activated the transcription factor NF-kappaB, and inhibition of NF-kappaB activation using a super-repressor IkappaB-alpha mutant was found to block the survival-promoting activity of the neurotrophins. Reporter gene assays and immunoblot experiments revealed that the neurotrophins also up-regulated the expression of Bcl-2, at both the transcriptional and protein levels. Overexpression of the super-repressor IkappaB-alpha mutant prevented this induction of Bcl-2 expression. Moreover, overexpression of either Bcl-2, alone, or the RelA subunit of NF-kappaB, alone, protected neurons from Tat-induced apoptosis. These findings suggest that the activation of NF-kappaB by neurotrophic factors may promote survival of neurons exposed to Tat, via regulation of anti-apoptotic genes including Bcl-2.

Regulation of cell death and survival in intestinal intraepithelial lymphocytes

Intraepithelial lymphocytes (IEL) of the small murine bowel represent a unique population of mostly CD8(+) T lymphocytes that reside within the epithelial cell layer of the intestinal mucosa. The close interaction with epithelial cells appears to be crucial for IEL survival since isolation and ex vivo culture induces massive apoptosis in this lymphocyte population. Here, we provide evidence that this form of IEL cell death may be mediated at least in part by endogenously produced glucocorticoids since adrenalectomy or treatment of mice with a glucocorticoid receptor antagonist significantly enhanced ex vivo survival of IEL. We further demonstrate that ex vivo activation of IEL induces upregulation of anti-apoptotic gene products, compensates for the lack of survival cytokines and rescues from apoptotic cell death. Thus, similar to thymocytes and T cell hybridomas, IEL survival may be regulated by the antagonistic action of TCR activation and glucocorticoids.

Western and Chinese antirheumatic drug-induced T cell apoptotic DNA damage uses different caspase cascades and is independent of Fas/Fas ligand interaction

Spontaneous or therapeutic induction of T cell apoptosis plays a critical role in establishing transplantation tolerance and maintaining remission of autoimmune diseases. We investigated the mechanisms of apoptosis induced by Chinese and Western antirheumatic drugs (ARDs) in human T cells. We found that hydroxychloroquine, Tripterygium wilfordii hook F, and tetrandrine (Tet), but not methotrexate, at therapeutic concentrations can cause T cell death. In addition, Tet selectively killed T cells, especially activated T cells. Although ARD-induced cytotoxicity was mediated through apoptotic mechanisms, Fas/Fas ligand interaction was not required. We further demonstrated that the processes of phosphatidylserine externalization and DNA damage along the ARD-induced T cell apoptotic pathway could operate independently, and that selective inhibition of DNA damage by caspase inhibitors did not prevent T cells from undergoing cell death. Moreover, we found that Tet- and Tripterygium wilfordii hook F-induced T cell DNA damage required caspase-3 activity, and hydroxychloroquine-induced T cell DNA damage was mediated through a caspase-3- and caspase-8-independent, but Z-Asp-Glu-Val-Asp-fluomethyl ketone-sensitive, signaling pathway. Finally, the observation that ARD-induced activation of caspase-3 in both Fas-sensitive and Fas-resistant Jurkat T cells indicates that Fas/Fas ligand interaction plays no role in ARD-induced T cell apoptosis. Our observations provide new information about the complex apoptotic mechanisms of ARDs, and have implications for combining Western and Chinese ARDs that have different immunomodulatory mechanisms in the therapy of autoimmune diseases and transplantation rejection.

Monomeric IgE stimulates signaling pathways in mast cells that lead to cytokine production and cell survival

Although IgE binding to mast cells is thought to be a passive presensitization step, we demonstrate herein that monomeric IgE (mIgE) in the absence of antigen (Ag) stimulates multiple phosphorylation events in normal murine bone marrow-derived mast cells (BMMCs). While mIgE does not induce degranulation or leukotriene synthesis, it leads to a more potent production of cytokines than IgE + Ag. Moreover, mIgE prevents the apoptosis of cytokine-deprived BMMCs, likely by maintaining Bcl-X(L) levels and producing autocrine-acting cytokines. The addition of Ag does not increase this IgE-induced survival. Since IgE concentrations as low as 0.1 microg/ml enhance BMMC survival, elevated plasma IgE levels in humans with atopic disorders may contribute to the elevated mast cell numbers seen in these individuals.